In, for example, the semiconductor manufacturing industry, it is important to control the atmosphere (the process environment) in which wafers are manufactured. The wafers are desirably manufactured in a controlled environment, as undesirable or varying levels of organic contaminants can result in device and/or equipment failure.
Levels of contaminating organic material in the parts per trillion (ppt) to parts per billion (ppb) range, which corresponds to a partial pressure of 10−9 mbar to 10−6 mbar, do not, in general, result in device or equipment failure. However, if the levels of organic contaminants become much higher than this, failures may result. In order to control the process environment, it is necessary to monitor the levels of organic contaminants present. In particular, some processes are sensitive to contaminant material in the low ppb range, and so for these processes it is desirable to monitor the level of contaminant materials in the ppt range. However, such monitoring processes are costly and it is difficult to determine an accurate value for the total organic compounds (TOC) present at such low contaminant levels. In addition, many fabrication processes are tolerant of light saturated hydrocarbons, such as methane (CH4) and ethane (C2H6), which have particularly low reaction probabilities with most surfaces and therefore do not take part in the various contamination inducing reactions.
In vacuum based process environments, TOC levels are often determined using mass spectrometry, as a mass spectrometer is capable of measuring contamination levels of the order of ppt. However the interpretation of such measurements is often complicated by effects such as mass spectral overlap, molecular fragmentation and background effects, for example.
Although mass spectrometers can be used in process environments operating at ambient pressure or above, additional vacuum and sample handling systems are required, which make such instruments very expensive. Under such conditions, it is preferred to use gas chromatographic techniques to monitor the TOC levels present in the process environment. However, in order to monitor contaminants in the ppt range it is necessary to fit the gas chromatogram with a gas concentrator.
It should be noted that although mass spectrometry and gas chromatography are able to detect ppt levels of TOC, their ability to differentiate the presence of the process-tolerant light hydrocarbons referred to above from the more harmful organic compounds is limited, which makes it difficult to determine the total levels of damaging hydrocarbons in the process environment.
In addition, because the use of either mass spectrometric or gas chromatographic techniques for determining the TOC levels present in process environments requires specialist equipment, they tend to be rather expensive and are typically only used as Point of Entry (POE) monitors for the whole facility rather than the more useful Point of Use (POU) monitors.
Hydrocarbons, including light hydrocarbons such as methane (CH4) and ethane (C2H6), have been routinely monitored using common tin oxide (SnO2) based sensor devices. These sensors typically operate under atmospheric pressure to detect target gases in the range from tens of ppb (parts per billion) to several thousand ppm (parts per million). This type of sensor works effectively within these ranges by providing a linear output signal that is directly proportional to the quantity of target gas within the monitored environment. Although these sensors are suitable for monitoring contaminant levels within ambient environments, they do not lend themselves for applications with sub-atmospheric processing environments such as those encountered within semiconductor processing environments. Under such vacuum conditions the SnO2-type of sensor will suffer from reduction of the active oxide content leading to signal drift and non-response after a period of time.
Wet potentiometric titration procedures and cyclic voltammetry are frequently used to monitor the levels of known contaminant species in solutions. All of these processes take place in the liquid phase and use reversible electrode processes, mostly using water as the major constituent of the solvent and use electrons to directly effect oxidation/reduction. Gas phase electrochemistry is confined to the areas of electrochemical sensors, both potentiometric and amperometric, and solid oxide fuel cells neither of which use any kind of titration reaction.
It will therefore be appreciated that, in contrast to the wet titrametric procedures of the prior art, the sensor of the present invention facilitates the titrametric determination of trace organic contaminants in the gaseous phase using a solid state electrolyte.
Chemical sensors comprising solid state electrolytes such as oxygen anion conductors, or silver or hydrogen cation conductors, have been used to monitor levels of oxygen, carbon dioxide, and hydrogen/carbon monoxide gas present in a process environment and are described in United Kingdom patent application number 0308939.8 and GB 2,348,006A, GB 2,119,933A respectively. Such sensors are generally formed from an electrochemical cell comprising a measurement electrode, a reference electrode and a solid state electrolyte of a suitable ionic conductor disposed between and bridging said electrodes.
For example, the gas monitor of GB 2,348.006A comprises a detection electrode containing a silver salt having an anion, which corresponds to the gas to be detected, a silver ion conducting solid state electrolyte and a reference silver electrode. The gas monitor can be used to detect gases such as carbon dioxide, sulphur dioxide, sulphur trioxide, nitrogen oxides and halogens through the suitable choice of the appropriate anion.
For the oxygen sensors of United Kingdom patent application number 0308939.8, the solid state electrolyte conducts oxygen anions and the reference electrode is generally coated or formed from a catalyst that is able to catalyse the dissociative adsorption of oxygen and is positioned within a reference environment, in which the concentration of oxygen adjacent the reference electrode remains constant.